Molybdenum is a low-resistivity refractory metal that can potentially replace tungsten as a material in memory, logic chips, and other devices using polysilicon-metal gate electrode structures. A thin film containing molybdenum can also be used in some organic light emitting diodes, liquid crystal displays, and also in thin film solar cells and photovoltaics. A thin molybdenum film can be used as a barrier film.
Various precursors and vapor deposition techniques have been used to deposit thin molybdenum films. Precursors include inorganic and organometallic reagents and vapor deposition techniques can include chemical vapor deposition (CVD) and atomic layer deposition (ALD) as well as a number of modifications such as UV laser photo-dissociation CVD, plasma assisted CVD, and plasma assisted ALD. The CVD and ALD processes are being increasingly used because they can give excellent conformal step coverage on highly non-planar microelectronics device geometries, however the costs and complexity of plasma assisted deposition and high temperature deposition systems can increase production costs and tool costs. High temperature processes can also damage previously deposited or underlying structures.
In a typical CVD process, the precursors are passed over an optionally heated substrate (e.g., a wafer) in a low pressure or ambient pressure reaction chamber. The precursors react and/or decompose on the substrate surface creating a thin film of deposited material like molybdenum. Volatile by-products are removed by gas flow through the reaction chamber. Some metal films are formed in a CVD process by supplying two or more gases to a reaction chamber with reaction of the gases leading to the deposition of the metal on the substrate. The deposited film thickness and uniformity depends on coordination of many parameters such as temperature, pressure, gas flow rates and mixing uniformity, chemical depletion effects, and time.
Refractory metal films have been deposited on substrates in a CVD process comprising heating in an enclosed chamber a substrate like silicon dioxide to a temperature of about 500° C. to 800° C., treating the heated surface with a vaporized substance like molybdenum hexafluoride for a brief period of time to increase the adherence of the surface to a molybdenum layer to be subsequently deposited, purging all the unreacted molybdenum hexafluoride from the chamber, and then depositing a molybdenum film by mixing hydrogen with some newly vaporized molybdenum hexafluoride to thereby reduce the molybdenum hexafluoride, generate HF(g), and deposit some of the molybdenum on the heated surface. The high temperatures for this deposition makes the processing equipment complex and consumes thermal budget for temperature sensitive devices. Further, the toxicity of HF(g) and associated abatement and safety equipment for handling HF(g) makes this process expensive and complex.
Smooth, low resistivity molybdenum films with good step coverage can be deposited on substrates by chemical vapor deposited (CVD) at high temperatures of about 700° C. using MoOCl4 or MoCl5 as molybdenum precursor and H2 as a reducing gas. These high temperature molybdenum films are useful, but lower deposition temperatures would be even more beneficial because it would consume less of the thermal budget of the materials used to make a device like a DRAM or photovoltaic, and because less expensive and complex equipment could be used to make the films. As the temperature of the substrate during the above deposition process was lowered below 700° C., a reaction temperature cutoff was observed that was about 550° C. for both MoOCl4 and MoCl5. Near this temperature, film roughness increased, film resistivity increased, and film deposition rate decreased and eventually ceased below the cutoff temperature. This cutoff temperature also limited the step coverage performance of the molybdenum film. Smooth, low resistivity molybdenum films with good step coverage are highly beneficial qualities of thin films used in semiconductor device manufacturing.
There is a continuing need to make molybdenum metal films and coatings on a variety of substrates at lower deposition temperatures and without complicated and expensive heating and vapor abatement equipment.